A fuel cell is a power generation system for producing electrical energy through the electrochemical redox reaction of an oxidant and a fuel such as hydrogen or a hydrocarbon-based material such as methanol, ethanol, natural gas, or the like.
The fuel cell can be classified as a phosphoric acid type, molten carbonate type, solid oxide type, polymer electrolyte type, or an alkaline type depending upon the kind of electrolyte used. Although each fuel cell basically operates in accordance with the same principles, the kind of fuel, the operating temperature, the catalyst, and the electrolyte may be selected depending upon the type of cell.
Recently, a polymer electrolyte membrane fuel cell (PEMFC) has been developed in which the power characteristics are superior to that of conventional fuel cells, the operating temperature has been lowered, and the starting and response characteristics are faster. It has advantages in that it can be applied to a wide variety of fields such as transportable electrical power sources for automobiles, distributed power such as for a house and a public building, and a small electrical power source for an electronic device.
The basic system of the PEMFC is essentially composed of an electricity generating element (a stack), a reformer, and a fuel supplier. The electricity generating element forms the body of the fuel cell, and the fuel supplier provides fuel to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies hydrogen gas to the stack. The electricity generating element generates electrical energy through the electrochemical reaction of the supplied hydrogen gas and oxygen.
On the other hand, the fuel cell may be a direct oxidation fuel cell (DOFC) in which liquid fuel is directly introduced to the electricity generating element. Such direct oxidation fuel cells include direct methanol fuel cells and do not have the reformer.
According to the fuel cell system described above, the stack actually generating the electricity has a structure in which a plurality of unit cells consisting of a membrane-electrode assembly (MEA) and a separator (also referred to as “bipolar plate”) are stacked. The membrane-electrode assembly is composed of an anode (referred to as “fuel electrode” or “oxidation electrode”) and a cathode (referred to as “air electrode” or “reduction electrode”) interposed by the polymer electrolyte membrane. The separator simultaneously acts as a path for supplying fuel required for the reaction to the anode and oxidant to the cathode, as well as a conductor for connecting the cathode of the membrane-electrode assembly to the anode of the neighboring membrane-electrode assembly.
In this process, an electrochemical oxidation of the fuel occurs on the anode, and an electrochemical reduction of oxygen occurs on the cathode, and as a result of the transfer of the electrons generated, electrical energy, heat, and water are obtained.
Protons generated at the anode are reacted with an oxidant supplied to the cathode on the surface of a platinum (Pt) catalyst to produce water. However, the platinum catalyst has a high cost, and therefore research has been devoted to a catalyst which can perform well using smaller quantities of expensive metals.